Geoscience Reference
In-Depth Information
reserve the same terms for characterizing electron acceptors. In addition,
the different possibilities combine and the microorganisms present
a very wide variety in functioning. But only some major life modes
pertain to the soil, in particular:
Chemo-litho-autotrophic
organisms
oxidize inorganic substances;
some are responsible for nitrification, others for the oxidation
of sulphur to sulphate, which corresponds to production of
sulphuric acid and result, for example, in the decay of our
monuments polluted by the residues of sulphur-containing
gasoline.
Chemo-organo-heterotrophic
organisms decompose organic
substances that are, for them, electron source, energy source and
carbon source all at the same time; they simultaneously search
for electron acceptors and reduce Fe
3+
to Fe
2+
for example, which
has various consequences in soils.
12.3.2
Decomposition of Organic Matter (Lovley 1995)
Fresh organic matter is made up of complex molecules that are
broken up or transformed to sugars, amino acids, fatty acids and
mono-aromatic compounds through fermentation processes involving
hydrolytic enzymes. The resulting short-chain compounds are then
oxidized. Oxidization reactions are exothermic and must be able to take
place spontaneously. Actually, they are catalysed by microorganisms
that recover part of the energy released. We have seen that an electron
acceptor is necessary. When air is present, this acceptor is oxygen, the
most powerful oxidant known:
The sequence of reduction reactions
¼ O
2
+ e
-
+ H
+
Æ ½ H
2
O
CO
2
Fermentable sugars
(glucose, cellulose,
hemicellulose), proteins,
organic acids, alcohol
(ethanol), phenol, etc.
e
-
O
2
H
2
Fig. 12.6
Decomposition of organic matter in presence of oxygen (respiration).
